Udp2raw Tunnel is a tunnel which turns UDP Traffic into Encrypted FakeTCP/UDP/ICMP Traffic by using Raw Socket, helps you Bypass UDP FireWalls(or Unstable UDP Environment). It can defend Replay-Attack and supports Multiplexing. It also acts as a Connection Stabilizer.

When used alone,udp2raw tunnels only UDP traffic. Nevertheless,if you used udp2raw + any UDP-based VPN together,you can tunnel any traffic(include TCP/UDP/ICMP),currently OpenVPN/L2TP/ShadowVPN and tinyfecVPN are confirmed to be supported.

Udp2raw Tunnel Features

Send/Receive UDP Packets with ICMP/FakeTCP/UDP headers

ICMP/FakeTCP headers help you bypass UDP blocking, UDP QOS or improper UDP NAT behavior on some ISPs. In ICMP header mode,udp2raw tunnel works like an ICMP tunnel.

UDP headers are also supported. In UDP header mode, it behaves just like a normal UDP tunnel, and you can just make use of the other features (such as encrytion, anti-replay, or connection stalization).

Simulated TCP with Real-time/Out-of-Order Delivery

In FakeTCP header mode,udp2raw tunnel simulates 3-way handshake while establishing a connection,simulates seq and ack_seq while data transferring. It also simulates following TCP options: MSS, sackOk, TS, TS_ack, wscale.Firewalls will regard FakeTCP as a TCP connection, but its essentially UDP: it supports real-time/out-of-order delivery(just as normal UDP does), no congrestion control or re-transmission. So there wont be any TCP over TCP problem when using OpenVPN.

Encryption, Anti-Replay

Encrypt your traffic with AES-128-CBC.

Protect data integrity by HMAC-SHA1 (or weaker MD5/CRC32).

Defense replay attack with an anti-replay window, smiliar to IPSec and OpenVPN.

Failure Detection & Stabilization (Connection Recovery)

Conection failures are detected by heartbeats. If timed-out, client will automatically change port number and reconnect. If reconnection is successful, the previous connection will be recovered, and all existing UDP conversations will stay vaild.

For example, if you use udp2raw + OpenVPN, OpenVPN won’t lose connection after any reconnect, even if network cable is re-plugged or WiFi access point is changed.

Other Features

Multiplexing One client can handle multiple UDP connections, all of which share the same raw connection.

Multiple Clients One server can have multiple clients.

NAT Support All of the 3 modes work in NAT environments.

OpenVZ Support Tested on BandwagonHost VPS.

Easy to Build No dependencies.To cross-compile udp2raw tunnel, all you need to do is just to download a toolchain,modify makefile to point at the toolchain,run make cross then everything is done.(Note:Pre-compiled binaries for Desktop,RaspberryPi,Android,some Openwrt Routers are already included in Releases)

(The above commands need to be run as root. For better security, with some extra steps, you can run udp2raw tunnel as non-root)

Server Output:

Client Output:

Now,an encrypted raw tunnel has been established between client and server through TCP port 4096. Connecting to UDP port 3333 at the client side is equivalent to connecting to port 7777 at the server side. No UDP traffic will be exposed.

Iptables rules,-a and -g

This program sends packets via raw socket. In FakeTCP mode, Linux kernel TCP packet processing has to be blocked by a iptables rule on both sides, otherwise the kernel will automatically send RST for an unrecongized TCP packet and you will sustain from stability / peformance problems. You can use -a option to let the program automatically add / delete iptables rule on start / exit. You can also use the -g option to generate iptables rule and add it manually.

--cipher-mode and --auth-mode

It is suggested to use aes128cbc + hmac_sha1 to obtain maximum security. If you want to run the program on a router, you can try xor + simple, which can fool packet inspection by firewalls the most of time, but it cannot protect you from serious attacks. Mode none is only for debugging purpose. It is not recommended to set the cipher-mode or auth-mode to none.

--seq-mode

The FakeTCP mode does not behave 100% like a real tcp connection. ISPs may be able to distinguish the simulated tcp traffic from the real TCP traffic (though it’s costly). seq-mode can help you change the seq increase behavior slightly. If you experience connection problems, try to change the value.

--lower-level

--lower-level allows you to send packet at OSI level 2(link level),so that you can bypass any local iptables rules. If you have a complicated iptables rules which conflicts with udp2raw tunnel and you cant(or too lazy to) edit the iptables rules,--lower-level can be very useful. Try --lower-level auto to auto detect the parameters,you can specify it manually if auto fails.

Manual format if_name#dest_mac_adress,ie:eth0#00:23:45:67:89:b9.

--keep-rule

Monitor iptables and auto re-add iptables rules(for blocking kernel tcp processing) if necessary.Especially useful when iptables rules may be cleared by other programs(for example,if you are using openwrt,everytime you changed and commited a setting,iptables rule may be cleared and re-constructed).

--conf-file

You can also load options from a configuration file in order to keep secrets away from ps command.

For example, rewrite the options for the above server example (in Getting Started section) into configuration file:

server.conf

-s
# You can add comments like this
# Comments MUST occupy an entire line
# Or they will not work as expected
# Listen address
-l 0.0.0.0:4096
# Remote address
-r 127.0.0.1:7777
-a
-k passwd
--raw-mode faketcp

Pay attention to the -k parameter: In command line mode the quotes around the password will be removed by shell. In configuration files we do not remove quotes.

Then start the server with

./udp2raw_amd64 --conf-file server.conf

--fifo

Use a fifo(named pipe) for sending commands to the running program. For example --fifo fifo.file.

At client side,you can use echo reconnect >fifo.file to force client to reconnect.Currently no command has been implemented for server.